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PAPER NO: 2

Computational investigation of the influence of fly ash

properties on the erosive wear behaviour of boiler grade

steel components

Amrita Kumari1, SK Das2 and PK Srivastava1

1 Birla Institute of Technology, Ranchi (Deoghar Campus) 835 215,India

2 CSIR-National Metallurgical Laboratory, Jamshedpur 831 007,India

AbstractA theoretical model has been developed embodying the ductile

erosion mechanisms involving cutting wear, plastic deformation and

surface temperature on the erosion response of typical boiler grade

steels. The parametric sensitivity of erosion response of these steel

grades as a function of particle impact velocity, angle of impingement

and steel surface operational temperature have been investigated which

also accounts for particle properties such as hardness (silica content)

and shape(angularity). The investigation demonstrated that a minor rise

in the fly ash hardness can considerably enhance the erosion rate of the

steel surface signifying that hardness of fly ash can be a crucial

parameter for characterising ductile erosion potential of various boiler

1

grade steels. The effect of fly ash angularity (shape) on the erosion

behaviour is also studied. The erosion resistance of the surface is

found to be dependent on the steel composition, specifically the amount

chromium content and tensile properties (yield strength) of the steel

Keywords: Mathematical model, ductile erosion, particle hardness,particle shape, tensile properties, Chromium content, boiler grade

steels

1. Introduction The generation of electricity in India relies largely on coal-

fired plants. However, most of Indian coal is of low quality with a

significant component of largely incombustible mineral matter. This is

made up of large quantities of extremely abrasive particles in the form

of silica/quartz (SiO2) and alumino-silicates. During the combustion of

coal inside the boiler, the mineral matter may undergo chemical changes,

such as loss of water of crystallization. It may also undergo physical

changes: the minerals within the coal itself may accrue to form hollow

spheres. These changes lead to the production of ash. Part of this

consists of relatively large particles, which drop to the bottom of the

boiler, where they are collected and removed by conveyors. Other parts

of the ash are present as relatively small particles and are carried

along with the combustion gas as it flows through the system. This is

known as fly ash. Repeated bombardment of the boiler steel

components by fly ash cause gradual removal of material by solid-

particle erosion. However, the problem appears to be generic and is

encountered in all coal fired power plants. The loss of small amounts of

material due to erosive wear is enough to cause serious damage and

reduce working lifetime of steel components. In fact, more than 25% of

all boiler tube failures worldwide are caused by fly ash erosion. In

cases of sever erosion, the components get perforated pre-maturely. As a

consequence, the components may fail once they lose their structural

integrity. The resulting penalty is not only the cost of replacing the

2

components but also the cost of stoppage of power production. It is,

therefore, industrial importance to be able to predict the rate of

erosion of the coal fired boiler components to evolve strategies for

preventive maintenance or replacement of these components to avoid

forced outages. Fly ash erosion is thus of great importance to the

economy of India , especially within the power generation field.

The two most important constituents of fly ash are, first, silica

(SiO2) and then alumina (Al2O3). It has been illustrated that 1 “free”

crystalline alumina mineral is hard and highly abrasive. However, in the

majority of pulverized coal ashes, alumina is present in a softer

“combined” form of alumino-silicate glass with some mullet needles

dispersed in the glassy matrix. In addition to silica content (particle

hardness), particle shape (angularity) plays a significant role in

enhancing the erosion potential. The erodent shape is an important

property but its effect is difficult to quantify for natural particles.

One of the most intriguing aspects of erosion is the influence of

erodent particle shape, which has been studied recently because of its

significant effect on erosion rate. It has been observed by few

investigators that angular particles produce a significantly higher

erosion rate than spherical particles of the same size. It is important

to establish the influence of particle size on the erosion rate of

ductile materials, since fly ash particles have a broad size

distribution.

The impact parameters that affect erosion damage are mainly impact

angle, velocity, size, shape and properties of the particles under

consideration. The mechanical properties of a material are also a

predominant parameter that affects erosion mechanisms. The impact

parameters that affect erosion damage are mainly impact angle, velocity,

size, shape, hardness and properties of the particles under

consideration. The shape of the particle is the manifestation of the

angularity of the erodent and another property namely hardness of the

3

particle, which is directly linked with. The overall erosion resistance

is found to be a critical function of the weight percent of the Chromium

content in the boiler grade steels which essentially exhibit superior

tensile properties the percentage of silica content, add to the effect

of increasing erosion rate. Higher the silica content implies higher

particle hardness. The erodent shape is an important property but its

effect is difficult to quantify for natural particles. The mechanical

properties of a material are also a predominant parameter that affects

erosion mechanisms. The material hardness is often representative of

mechanical properties. On the other hand, erosion damage can widely vary

depending on impingement angle and the mechanical properties of the

material.

The issue of solid particle erosion has been addressed by various

investigators which deals with erosion behaviour at room and elevated

temperatures. Many parameters are now known to influence erosion

behaviour. The magnitude and direction of a particle's impact and

rebounding velocity depend upon the conditions at impact and the

particular particle-surface material combination. The restitution

behaviour is a measure of the momentum lost by the particle at impact as

such and it corresponds to the work done on the target surface, which,

in turn, is a measure of the extent of erosion suffered by the material

of the target surface.. Grant and Tabakoff 2developed empirical

correlation of the velocity restitution coefficients for sand particles

impacting on 410 stainless steel. Meng and Ludema 3 have reviewed some

of the erosion models that have been developed since the inception of

analytical erosion model of Finnie 4.These models include a variety of

parameters that influence the erosion of a ductile material envisaging

the mechanism of erosion. The governing equations proposed by Finnie4 to

calculate erosion of surfaces by solid particles in terms of volumetric

removal of materials are as follows:

4

(1)

(2)

where vp is the volume of material removed by a single abrasive grain of

particle, m is mass of single particle, V is velocity of particle, P is

constant of plastic flow stress, is the ratio of depth of contact to

the depth of cut and k the ratio of vertical to horizontal force

component respectively Here, is the impact angle. It was assumed that

the maximum erosion occurs, when tan 0 = k /6 which defined a critical

impact angle 0. The critical angle 0 is the impact angle at which the

horizontal velocity component has just become zero v, when the particle

leaves the body i.e. the impact angle above which the residual

tangential speed of the particle equals zero. Subsequently Bitter5

calculated total erosion rate which is the sum of erosion due to cutting

mechanisms and deformation mechanism without the effect of temperature

by the following derived equations

vT = vD + vC(3)

(4)

(5)

(6)

5

where, vT is total volume erosion rate, vD is volume of material removed

by deformation mechanism, vC is the volume of material removed by

cutting mechanisms , M is total mass of impinging particle, K is

velocity component normal to surface below which no erosion takes place

in certain hard materials, K1 is proportionality constant and C is a

model constant.

Subsequent experimental and computational investigations6,7

contributed for the improved understanding of the mechanisms of erosion,

however, the detailed mechanism of erosion are yet to be fully explored.

The high temperature erosion behaviour is associated with complex

mechanism due to the variations in tensile properties, materials

properties and erosion-oxidation interaction after a prolonged exposure

and significant growth of oxide scale on the component surface. Reported

erosion test results8 of various materials up to 6000C demonstrate

varying tendencies of erosion pattern depending on materials. It has

been observed that erosion rates of steels impacted at low angles

increase at elevated surface temperatures. Also, it has been observed

that rates of erosion may vary depending on the nature and composition

of steel. Winter and Hutchings9 identified the two regimes of deformation

as ploughing and micro-cutting. Ploughing was not favoured when the

particle rolled over instead of sliding along the surface. Rolling

caused the cutting edge of the particle to penetrate deeply into the

metal surface instead of performing a scooping action. They further

reported experimental investigations to determine the erosion mechanism

of particle orientation during oblique impact of angular particles on

lead and mild steel targets. It has been also reported by them that for

particle incidence angles close to 900, erosion occurred predominantly by

plastic deformation of the target. Levy10 carried out experimental

morphological investigations on eroded metal surfaces using scanning

electron microscopy at high magnifications. He observed that the loss of

6

material from an eroded metal surface occurred by a combined cutting

wear (micromachining) and extrusion-forging mechanism.

The ever-increasing capability of computers has led to the

development of several models for gas-particle flows to simulate the

discrete phase flow behaviour. Computational fluid dynamics (CFD)

simulations11 of flue gas laden with fly ash ( discrete phase) was

carried out to predict erosion behaviour of the economisers in coal-

fired boilers and subsequently Lee et.al12 used Lagrangian particle

tracking CFD analysis to predict erosion of boiler tubes. These CFD

models are highly computation intensive and inherently lack appropriate

integration of materials model to capture the complex erosion mechanisms

although particle impact parameters and adjoining fluid flow fields were

modelled accurately. Deng et al.13, for the first time, made experimental

measurements of particle rotation. The erosion behaviour of impacted fly

ash particles on coal fired boiler components has been studied at

elevated temperature to characterize the erosion rate(mg of steel eroded

/ Kg of erodent) as a function of various operating parameters 14-16

Silica is known to be highly abrasive and its quantity in the fly

ash is of critical importance on the erosion potential of coal fired

boiler components. The experimental and theoretical investigations

specifically addressing characterization of erosion behaviour of fly ash

on different boiler grade steel with varying degree of hardness (% of

silica content) is rather scanty in the available literature. An

analytical model was reported17 in the literature to characterize the

erosion behaviour of ash particle embodying composite mechanism of both

cutting wear and plastic deformation to predict the erosion rate as a

function of particle velocity, impingement angle, density of the target

material and its tensile properties at normal temperature. Necessary

modifications14,15 have been subsequently incorporated to account for the

thermal effects on the erosion behaviour at elevated temperature of the

surface. An erosion index has been defined, which relates the variation

7

of the erosion rate to the silica content. Sensitivity of silica content

in the ash on the erosion rate has been investigated for various grades

of steel at room and elevated temperature. In this study the earlier

models14,15 have been improved to provide a predictive framework to

analyse the effect of particle hardness (percentage of silica content)

along the particle angularity (shape). The erosion sensitivity of

various particle impact parameters, namely, impact velocity, angle of

impingement and variation of steel surface (surface) temperature have

also been studied in conjunction with hardness (silica content) and

shape of the ash particles.

2. Ductile Erosion Model of Fly Ash The magnitude of the erosive wear is quantified by the volume or

mass of the material that is removed by the action of the impacting

particles. It is evident from the earlier studies 9, 15, 17, 18 that there

are three important phenomena by which metal can be removed at elevated

temperature by the process of erosion. These are primarily attributed

to, namely, material erosion due to cutting wear (micro machining

action), repeated plastic deformation wear (extrusion-forging mechanism)

and deterioration of tensile properties of the steel component at

elevated temperature. The erosion at room temperature is due to cutting

wear and plastic deformation wear whereas at elevated temperature along

with these mechanisms effect of temperature on tensile properties is

invoked as an additional parameter. In principle, the erosion rate is

expected to be a function of following parameters inclusive of the

above:

Ash particle velocity

Ash particle impingement angle

Average density of ash particles

Hardness of the particle (% silica content)

Angularity (shape) of the particle

8

Density of target material

Yield stress of the steel component

Temperature of the steel component

2.1 Analytical treatment of Cutting Wear

At the impact point on the steel surface, fly ash particle loses a

fraction of its kinetic energy to the target material in the form of

heat and energy for deformation of the surface. Very high levels of

shear strain may be induced in the material at this point. When the

shear strain exceeds the elastic strain limit of the target material,

the particle penetrates the surface of the material and ploughs along

the surface removing material.

During wear process, it may be assumed that the stresses acting at

the contact point are constant. Consequently, the ash particle

penetrating the surface of the material has to overcome the material's

resistance to deformation. The equation of motion for the depth of

penetration, h, of a particle of mass mp and diameter dp as it penetrates

through the surface of a material can be formulated in the form of

force balance differential equation 9,14,1518

(7)

where, t is the time, y the yield stress of the target material, and C is

a particle shape factor equal to 3 for a sphere. The negative sign in

Eq. (7) signify that the material resists the penetrating action of the

impacting particle. The mass, mp, for a spherical particle is derived

from the following simple relationship:

mp = p

(8)

9

Substituting for the mass of the spherical particle given in Eq (8), the

Eq. (7) may be rewritten as:

(9)

When a particle strikes a steel surface (ductile material) with a

velocity V and at an angle of incidence, the initial rate at which the

particle penetrates into the material is equal to the normal component

of the impact velocity. Eq. (9) is integrated using the initial

condition that at t=0, (dh/dt) = Vsin, and the following equation is obtained:

(10)

The implication of plus sign in Eq. (10) corresponds to an increase in

the depth of penetration and the minus sign corresponds to a decrease in

the depth of penetration. The maximum depth of penetration, hmax, occurs

when (dh/dt) =0, and is given by the equation

h3max = (11)

Since the volume of material that is cut away from the target surface

by the impacting particle is proportional to h3max, the mass of material

removed by a single particle is also be proportional to the value of

h3max. The mass of material eroded by cutting mechanism ‘mc’ by a single

impacting particle, thus, may be given by the following equation:

mc = Kcm h3max =

(12)

here Kc is a constant and m the density of target material. The erosion

rate due to cutting wear, defined as the ratio of the mass of the

material eroded from the target surface to mass of the impacting

particle, is

10

(13)

where, K1 is a model constant.

2.2 Analytical treatment of plastic deformation wear

Extrusion-forging mechanism can result in material loss during the

particle impingement on the surface. Once initially formed platelets are

extruded during particle impact, the loss of material from an eroding

surface may occur by a combined extrusion-forging mechanism. Platelets

are initially extruded from shallow craters made by the impacting

particle. Because of the high strain rates, adiabatic shear heating

occurs in the surface region immediate to the impact site. Beneath the

immediate surface region a work hardened zone forms, as the kinetic

energy of the impacting particles is enough to result in a considerably

greater force being imparted to the metal than is required to generate

platelets at the surface. When the surface is completely converted to

platelets and craters and the work-hardened zone reached its stable

hardness and thickness, steady state erosion begins. Subsurface cold-

worked zone acts as an anvil, thereby increasing the efficiency of the

impacting particles to extrude-forge platelets in the now highly

strained and most deformable surface region. This cross section of

material moves down through the metal as erosion loss occurs. In the

platelet mechanism of erosion, there is a localised sequential extrusion

and forging of metal in a ductile manner, leading to removal of the

micro segments thus formed.

During plastic deformation, the normal component of the particle's

kinetic energy9,14,15,18 is used to extrude-forge the material. The normal

component of the kinetic energy of the particle (KE1) is given by :

11

(14)

where dp and p are the particle diameter and density, respectively, and

V and the particle incident velocity and angle, respectively. The work

done (W┴) by the normal force (F┴) of the indenting particle in a

direction h normal to the surface from the time of surface contact until

penetration stops at a depth hmax is given by the

(15)

Sheldon and Kanhere20 formulated the following equation relating the

force F┴ and the diameter of the crater formed in the indented surface

F┴ = an (16)

where constants, n and a, are given as follows: , n = 2.0 and HV is Vickers hardness number of the

target surface eroded by particle impingement. Substituting Eq. (16)

into Eq. (15) yields

(17)

The depth of penetration, h, is related to the instantaneous crater

diameter and the particle diameter dp as

(18)

Eq. (18) is used to express the particle's depth of penetration in terms

of the instantaneous crater diameter. The integral in Eq. (17) may be

12

evaluated with respect to the instantaneous crater diameter. Equating

the work done during indention to the normal component of kinetic energy

given in Eq. (14), the following equation is obtained

(19)

The integral in Eq. (19), is evaluated and the maximum depth of

penetration is derived as

(20)

The dimensions of the crater formed by the impacting particle are

assumed to be all proportional to h3max, and since the amount of material

removed is nearly the full crater size, the mass of material removed by

a single particle is proportional to the value of h3max given in Eq.

(20). The mass of material removed ‘md’ by plastic deformation mechanism

by a single particle is given by the following equation

(21)

where Kp is a model constant and m is the density of the target

material. The erosion rate,p, due to plastic deformation is given by the

following equation

(22)

13

where K2 is also a model constant. The model constants are tuneable

parameters15,18.

3. Tribological characteristics of fly ash erosion3.1 The effect of surface temperature of steel

The erosion by fly ash of the boiler components consists of the

wear due to the cutting mechanism plus the wear due to the plastic

deformation mechanism. However, it is difficult to predict accurately

the separate contributions by each of the two mechanisms to the overall

material loss. Eq. (22), which was derived for the plastic deformation

wear, is similar to Eq. (13) for the cutting wear. The yield stress of a

metal can be related to the metal's hardness. Tabor19 proposed the

following empirical relationship between the yield stress and Vickers

hardness number

HV = 2.7(T)y (23)

The overall erosion rate, combining the cutting and plastic deformation

wear mechanisms, is then given as:

(24)

where K3 is a constant which is documented in the literature 3,6,7,19. From

the investigations carried out by various investigators 7,20,21 the erosion

rate due to solid particle impact depends upon the particle impingement

angle and the characteristics of the particle-wall combination for

modelling erosion by fly ash of ductile metal surfaces.

The effect of temperature on the erosion behaviour of boiler

components is of practical importance and an attempt was made to

14

functionally correlate the tensile properties of these materials at

elevated temperatures, which has been incorporated in the model. In the

present model, the following process and materials parameters are

considered for predicting erosion rate in the boiler components. The

temperature effect has been introduced in this model on the basis of the

observation that the erosion rate at an acute impingement angle

increases significantly with temperature suggesting that steel tends to

show behaviour more typical of a ductile material at elevated

temperatures. The yield stress (Kgf/mm2) and temperature (K)

functionality has been derived through a polynomial approximation for

various grades of steel on the basis of the available tensile property12,

22. The following expressions have been generated.

Carbon Steel

(25)

Cr-1Mo-V steel

(26)

2.25Cr-1Mo steel

(27)

12Cr-1Mo-V steel

(28)

304 steel

(29)

Alloy (Incoloy) 800

15

(30)

3.2 Effect of fly ash hardness

The erosion rate of a material depends on it’s abrasive hardness, more

precisely, on the material to abrasive hardness ratio. If material

hardness is lower than abrasive hardness, micro-cutting or surface

scratching may take place. If the material hardness is lower than the

abrasive hardness, clear removal of materials usually does not take

place and the entire process has the nature of fatigue. At high- energy

impact of abrasive particle, brittle rupture of materials and detachment

of grains or their fragments may take place. The constant K3 in Eq. (24)

may be replaced by the particle erosion index to generate the expression

for the overall erosion rate 14.

(31)

where Ke is a constant, Si is the mass fraction of silica contained in the

ash sample which directly incorporates the hardness of the ash particle

and Ie the erosion index of the fly ash which relates the variation of the

erosion rate to the hardness (silica content).

(32)

where ξ and ψ are coefficients related coal characteristics. The

values of ξ≈ 3.5 and ψ ≈ 4.95 are incorporated in the model14,15.

Substituting the equation (32) in equation (31), the expression for

overall erosion rate can be given as:

16

(33)

3.3 Effect of shape and angularity of fly ash

Shape, the first-order morphologic property, is used to

characterize the gross form of a particle and it is mostly defined in

terms of three perpendicular axes. Angularity, the second order

property, expresses the number and sharpness of corners on the particle

surface. Surface roughness, the third-order property, reflects the

number, size and sharpness of the asperities along the particle surface

and on the corners.

In order to incorporate the impact angle dependence of erosion

damage 22, a functional approximation has been made in terms of g( α ),

the ratio of erosion damage at arbitrary angles, to that at normal angle

. Erosion damage at arbitrary angles E(βi) can then be expressed as:

(34)

g(βi) denotes the impact angle dependence of normalized erosion expressed

by the two trigonometric functions and by initial material (Vicker’s )

hardness number Hv

(35)

(36)

where k1, k2 and k3 are adjustable parameters23 and detailed parametric

ranges and values are described elsewhere 23. Here K denotes a

particle property factor (designated as coefficient of angularity

exponent) to account for particle shape (angularity) during ballistic

impact. It is considered that K denotes an adjustable model parameter

17

for as one of the particle characteristics and whose values can be

adjusted in consonance with particle angularity24.

Table I and table II show the chemical composition and chemical

elemental details of Indian fly ash. Table III shows the chemistry of

some typical grades of steel used in boiler assembly. The model has been

implemented in a C++ computer code (designated as EROSIM-INTEGRATED)

which embodies the solid particle erosion mechanism due to cutting wear

and repeated plastic deformation in conjunction with fly ash particle

properties (hardness and angularity) and impact parameters. The erosion

behaviour at elevated temperature has been incorporated through the

derived functionality of the tensile property (yield stress) as a

function of temperature using earlier respective polynomial equations.

4.0 Results and DiscussionFigure 1 shows prediction of erosion model (mg of the surface

materials (steel) removed per Kg of impacted erodent) which has been

quantitatively validated with the published data [20] for 1.25Cr-1Mo-V

steel at impingement angle of 300 and at normal temperature (30 C). The

ductile erosion model predictions (order of magnitude) are found to be

in good agreement with the literature data. Availability of open domain

published data on erosion rate specifically incorporating the boiler fly

ash hardness (silica content in the fly ash) is rather scarce. Figure 2

shows the relative erosion rate of ash particle as a function of impact

velocity at two different particle hardness levels in consonance with

respective silica content, namely, 55% and 70% and at an impingement

angle of 300 .The surface temperature is at a standard temperature of

30ºC. The erosion behaviour shows a power law relationship with

increasing impact velocity which is attributed to the typical erosion

characteristic of any ductile material. Figure 3 depicts the erosion

rate as a function of impingement angle with impact velocity of 50 m/sec

for 1.25Cr-1Mo-V steel addressing two typical cases of particle hardness

18

(70% and 55% silica content). For particle hardness corresponding to 70%

of silica content in the ash, erosion loss is about 45 mg/Kg of erodent

at a particle impingement angle of 300 and impact velocity of 50 m/sec.

However, with a depletion of silica content to 55% the erosion rate

diminishes considerably (13 mg/Kg) with same impact velocity and impact

angle. This signifies that an increase in the hardness (silica content

of 15 %) of the fly ash has enhanced the erosion rate more than three

times under similar particle impact conditions. Figure 4 illustrates the

relative erosion rate of carbon steel as a function of elevated

temperature with silica contents of 55% and 70% respectively at an

impingement angle of 30° and particle impact velocities 50 and 30 m/sec.

From this figure, it may be observed that the erosion rate monotonically

increases as a function of elevated temperature irrespective of particle

hardness and ballistic impact parameters. Further, the conjoint action

of both particle hardness (silica 70%) and higher impact velocity (V =

50 m/sec) aggravated erosion rate significantly in comparison to other

cases depicted in the same figure. According to the ductile erosion

mechanism, the range of impingement angle 200 - 400 is critical in terms

of maximum erosion loss of the metallic material. This indicates that an

increase of about 15 % in the silica content (and corresponding increase

in the hardness) of the ash has a significant impact on the erosion

rate, which has increased more than 200 % under similar particle impact

and surface conditions. Figure 5 shows the erosion rate of 1.25 Cr-1Mo-V

steel as a function of elevated temperature with two different particle

hardness (namely, silica contents of 55% and 70%) at an impingement

angle of 30° for particle impact velocities of 50 and 30 m/sec. The

depletion of erosion rate with respect to carbon steel under similar

condition is primarily attributed to the superior tensile properties and

erosion resistant of 1.25 Cr-1Mo-V steel with respect to carbon steel.

Figure 6 depicts a comparative view of variation of erosion rate as a

function of impingement velocity for two different steel grades, namely,

19

carbon steel and 1.25Cr-1Mo-V steel with impingement angle of 30° at

normal temperature (30 C). The ash particle hardness is kept equivalent

to 70% of silica content. Similarly, figure 7 shows the comparative

study of erosion rate as a function of impingement angle for two

different steel grades, namely, carbon steel and 1.25Cr-1Mo-V steel with

impingement velocity 50 m/sec and at normal temperature (30 C). As

earlier, the ash particle hardness is kept invariant which is equivalent

to 70% of silica content.

Figure 8 shows effect of particle angularity on erosion rate as a

function of impingement angle, with an angularity exponent as 0.22 and

impact velocity of 50 m/sec for both carbon steel and 1.25Cr-1Mo-V

steel. The temperature of the surface is kept at normal temperature i.e

30C. Incorporation of the angularity effect of particle manifests higher

erosion rate even at higher impingement angles(reference to the results

shown in Fig.3). Figure 9 shows effect of particle angularity on erosion

rate as a function of impingement angle, with an angularity exponent as

0.22 and impact velocity of 50 m/sec for both for both 1.25 Cr-1Mo-V

steel and 12Cr-1Mo-V. The comparative erosion profiles demonstrate that

12Cr-1Mo-V steel has higher erosion resistance which may be attributed

to the higher Chromium content in the steel chemistry. Figures 10

depicts a comparative illustration of variation of erosion rate as a

function of impingement angle for two more boiler grade steels, namely,

12Cr-1Mo-V and Alloy 800 steel with the same angularity exponent 0.22.

The higher erosion resistance of Alloy 800 steel predicted by the

ductile erosion model is understandably attributed to the relatively

higher chromium content in the steel chemistry. Figure 11 shows

comparative illustration of erosion rate profiles as a function of

impingement velocity for both carbon steel and 1.25Cr-1Mo-V steel with

particle angularity exponent as 0.22. From this figure it is again

observed that the erosion rate exhibits a power law relationship with

the impact velocity of the particle. The erosion rate is also enhanced

20

(10% approximately) with the incorporation of particle angularity.

Figure 12 depicts a comparative erosion response of the effect of

particle angularity (0.22 and 0.33) on the erosion rate as a function of

impingement angle for a fixed impingement velocity being 50 m/s. It may

further be observed that the erosion rate for particle with 0.22

angularity exponent is lower with respect to the erosion rate with

higher angularity exponent (0.33) under similar particle ballistic

impact conditions. This indicates that there is an approximate 20 %

increase in the erosion rate as the angularity exponent has been

enhanced from 0.22 to 0.33 keeping other erosion parameters invariant.

Figure 13 shows the variation of erosion rate as a function of

angularity exponent of the particle for both carbons steel and 1.25Cr-

1Mo-V steel. The particle velocity and impingement angle considered are

50m/s and 30 deg respectively. The erosion rate is found to be

increasing with higher angularity exponent of the ash particle for both

carbon and 1.25Cr-1Mo-V steels which exhibits relatively linear

behaviour.

5. ConclusionA ductile erosion model incorporating the effects of ash particle

hardness and angularity has been developed to characterize the erosion

behaviour of some typical boiler grade steels and validated with the

published experimental data where ever feasible. The paucity of

published experimental or industrial data on actual fly ash (with

variation in silica content) on erosion response of various boiler grade

steel has been a limitation to accomplish comprehensive validation and

verification of the predictions. However, validation of order of

magnitude predictions has been carried out with the available published

data. The erosion behaviour as a function of various ballistic impact

parameters have been investigated in conjunction with particle hardness

(silica content) and angularity (shape). It has observed that the

erosion rate for all steel grades is a strong function of particle

21

hardness and angularity in conjunction with the conventional

tribological parameters. For low values of impingement angle, the

erosion rate increases with increase in the impingement angle, with the

maximum erosion rate occurring at an impingement angle range of 20-30

degree. Thereafter, the erosion rate decreases with further increase in

the impingement angle which is consistent with the ductile erosion

theory. Further the erosion rate at low impingement angles increases

significantly with increasing temperature but at high impingement angles

the effect of temperature is insignificant. This investigation also

illustrates the enhancement of erosion rate at elevated temperatures

because of deterioration of tensile properties of the surface material.

The variation of erosion rate shows consistent power law behaviour as a

function of rising particle impact velocity in conjunction with particle

physical properties such as hardness and shape. The predictions

emphasize that the hardness (silica content) and shape (angularity

exponent) of the erodent particle have considerable influence on the

erosion behaviour boiler grade of steels which needs extensive

experimental characterization using actual boiler fly ash as erodent.

The overall erosion resistance is found to be a critical function of the

weight percent of the Chromium content in the boiler grade steels which

essentially exhibit superior tensile properties with increasing Chromium

content. It may be concluded that the particle hardness and shape are

crucial erosion parameters which needs to be addressed for any solid

particle erosion investigations using appropriate methodology parameters

to elucidate realistic characterization of the phenomena.

Acknowledgement:

The author thankfully acknowledges the support provided by the

Council of scientific & industrial research (CSIR), New Delhi,

India, under the CSIR Network project (NWP 0027) for undertaking this

activity.

22

References[1] E. Raask : Mineral Impurities in Coal Combustion, Behaviour, Problems, and Remedial

Measures, Hemisphere Publishing Corporation, 1985.

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[3] H.C Meng and K.C Ludema: Wear , 1995,181, 443-457

[4]I Finnie: Wear, 1960, 142, 87-103.

[5] J.G.A Bitter: 1963, Wear 6, 5-21.

[6] Y.D Jun, W Tabakoff: Trans. ASME: J.Fluids Eng. 1994, 116 , 770-777.

[7] J, D. Fan, K Cen. Zhou and J. Jin: Chem. Eng. Commun. 1990, 95 , 75-

88.

[8] G.P. Tilly: Wear,1969, 14 : 63.

[9] I.M Hutchings and R.E. Winter: Wear, 1974, 27, 121-128.

[10] A. V. Levy: Wear, 1986, 180, 1-21.

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J. Eng. Gas Turbines Power, 1997,119, 709-716

[12] B.E Lee, C.A.J Fletcher and M Behnia: J. Eng. Gas Turbines Power, 1999,

121, 746-750.

[13] T Deng, M.S Bingley and M.S.A Bradley: Wear , 2004, 256, 1037-1049.

[14] SK Das, KM Godiwala , SP Mehrotra and PK Dey: High Temperature Materials

and Processes, 2006, 25 (5-6), 323-335

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2006, 31(5), 1-13

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Proceedings of International Conference on Industrial Tribology(ICIT-2006), Bangalore,

India, November 2006, Indian Institute of Science, 189-196,

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Calculation Methods, Pergamon Press, 1982.

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23

[21] G.L Sheldon, J. Maji and C.T.Crowe: Trans. ASME: J. Eng. Mater. Technol.,

1977, 99, 138- 142.

[22] Y. Shida and H. Fujikawa: Wear,1985, 103, 281-296

[23 ]Y.I. Oka, K. Okamura, T. Yoshida: Wear, 2009, 267, 129–135.

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24

Table –I Chemical composition of Indian fly ash\\\Constituent Compositional range

(%)

Silica (SiO2) 49-70

Alumina (Al2O3) 17-28Iron Oxide (Fe2O3) 5-10Calcium Oxide (CaO) 1-4Magnesium Oxide (MgO) 0.2-2Sulphur (SO3) 0.1-2

Table -II Chemical elements in the Indian fly ash

Sl.

no. Elements Range

1Aluminium

(Al)

15.167–

20.45

2 Calcium (Ca) 0.37–0.76

3 Iron (Fe) 4.447–6.562

4Manganese

(Mn)0.002–0.84

5Magnesium

(Mg)0.02–0.9

6Phosphorous

(P)0.06–0.3

25

Sl.

no. Elements Range

7Potassium

(K)0.14–1.8

8 Silicon (Si)27.413–

29.554

9 Sodium (Na) 0.07–0.71

10 Sulphur (S) 0.03–0.055

Table –III Chemical composition of some typical grades of

steel

SteelAmount in weight %

C Si Mn Ni Cr Mo VCarbon

Steel

0.22 0.28 0.65

1.25Cr-1Mo-

V

0.13 0.25 0.55 1.20 0.95 0.3

02.25Cr-1Mo 0.10 0.34 0.44 2.20 0.9812Cr-1Mo-V 0.19 0.33 0.59 11.4

0

0.87 0.2

8304 0.08 0.62 1.68 10.2

5

18.5

0Alloy 800 0.07 0.51 1.13 32.8 20.8

26

5 5

27

Fig. 1 Order of magnitude validation of model predictions of erosion

rate as a function of

Impingement velocity for 1.25 Cr-1Mo-V Steel at normal temperature (30

C)

28

(Particle Impingement Angle= 300)

Fig. 2 Relative effect of particle hardness (silica content )on the

erosion rate of Carbon steel as a function of impingement velocity at

normal temperature (30ºC).

Fig. 3 Variation of erosion rate for different particle hardness (silica

content) for

1.25 Cr-1Mo-V as a function of impingement angle at normal temperature

(30ºC)

29

(Hardness variation with silica contents=55%and 70%, Impingement velocity=50m/s)

Varition of erosion rate as function of substrate tem perature and hardness ( silica contents= 55% and 70%, im pingem ent angle=30 deg,

Velocities = 50 and 30 m /sec)

0

10

20

30

40

50

60

70

0 50 100 150 200 250 300 350Tem perature (°C)

Erosion Rate (m

g/kg)

SiO=70 %, V =50m /secCarbon Steel

SiO=70 %, V =30m /sec

SiO=55 %, V =50m /sec

SiO=55 %, V =30m /sec

Fig. 4 Variation of erosion rate for different particle hardness

(silica content) and

impact velocities of Carbon steel as a function of elevated temperature

0

5

10

15

20

25

30

35

0 50 100 150 200 250 300 350Tem perature (°C)

Erosion Rate (m

g/kg)

SiO= 70%, V=30m /sec

SiO= 55%, V=30m /sec

1.25Cr-1M o-V Steel

SiO= 55%, V=50m /sec

SiO= 70%, V=50m /sec

Varition of erosion rate as function of substrate tem perature and hardness silica contents= 55% and 70%, im pingem ent angle=30 deg,

Velocities = 50 and 30 m /sec

30

Fig. 5 Variation of erosion rate for different particle hardness

(silica content) and

impact velocities of 1.25Cr-1Mo-V steel as a function of elevated

temperature

Fig. 6 Variation of erosion rate with impact velocity for both Carbon

and

1.25Cr-1Mo-V steel at a normal temperature of 300

31

(Particle hardness with silica content 70 %, particleimpingement angle= 300)

Fig. 7 Variation of erosion rate with impact angle for both carbon steel

and 1.25 Cr-1 Mo-V steel at a normal temperature (particle hardness with

silica content 70%)

(Im pingem ent velocity=50m /s and angularity exponent=0.22)

0

20

40

60

80

100

120

0 20 40 60 80 100

Im pingem ent Angle (deg.)

Erosion

Rate (m

g/kg)

Carbon Steel

1.25 Cr-1M o-V

32

(Particle hardness with silica content 70 %, impingement velocity=50m/s)

Fig. 8 Variation of erosion rate for Carbon and 1.25Cr-1Mo-V steel as a

function of impingement angle at normal temperature (30ºC) with silica

content 70 %

010203040506070

0 20 40 60 80 100Im pingem ent Angle

Erosion

rate( m

g/kg)

1.25 Cr-1M o-V

12 Cr-1M o-V

(Im pingem ent velocity=50m /s and angularity exponent=0.22)

Fig. 9 Variation of erosion rate for 1.25Cr-1Mo-V and 12Cr- 1Mo-V steel

as a function

of impingement angle at normal temperature (30ºC) with silica content

70%

33

05101520253035404550

0 20 40 60 80 100Im pingem ent Angle (deg.)

Erosion

Rate (m

g/kg)

12 Cr-1M o-V

Alloy 800 Steel

(Im pingem ent velocity=50m /s and angularity exponent=0.22)

Fig. 10 Variation of erosion rate of 12 Cr-1Mo-v and Alloy 800steel 800

as a function

of impingement angle at normal temperature (30ºC) with silica content

70%

( silica content= 70%, Im pingem ent angle=30 deg, Erosion exponent = 0.22)

0

100

200

300

400

500

600

700

0 20 40 60 80 100Im pingem ent Velocity (m /s)

Erosion

Rate (m

g/kg)

Carbon Steel

1.25Cr-1M o-V

Fig. 11 Variation of erosion rate for Carbon and 1.25Cr-1Mo-V steelas a

34

function of impingement velocity at normal temperature (30ºC)

(Im pingem ent velocity=50m /s and angularity exponents=0.22 and 0.33)

020406080100120140

0 20 40 60 80 100Im pingem ent Angle (deg.)

Erosion

Rate (m

g/kg)

K=0.33K=0.22

Carbon Steel

Fig. 12 Variation of erosion rate of Carbon Steel for two different

angularity

exponents ( K values) at normal temperature 30 C

35

(Im pingem ent velocity=50 m /s and angle=30° , fly ash hardness = 70 % silica )

0

50

100

150

200

250

300

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Angularity Exponent

Erosion Rate (m

g/kg)

carbon steel

1.25Cr-1M o-V

Fig. 13 Variation of erosion rate for Carbon Steel and 12Cr-1Mo-V steel

as a

function of angularity exponent at normal temperature (30ºC)

36

Figure Captions

Fig. 1 Order of magnitude validation of model predictions of erosion

rate as a function of impingement velocity for 1.25 Cr-1Mo-V Steel

at normal temperature, (30 C)

Fig. 2 Relative effect of particle hardness (silica content) on the

erosion rate of Carbon steel as a function of impingement velocity

at normal temperature (30ºC).

Fig. 3 Variation of erosion rate for different particle hardness (silica

content) for

1.25 Cr-1Mo-V as a function of impingement angle at normal temperature

(30ºC)

Fig. 4 Variation of erosion rate for different particle hardness

(silica content) and

impact velocities of Carbon steel as a function of elevated temperature

Fig. 5 Variation of erosion rate for different particle hardness

(silica content) and

impact velocities of 1.25Cr-1Mo-V steel as a function of elevated

temperature

Fig. 6 Variation of erosion rate with impact velocity for both Carbon

and

1.25Cr-1Mo-V steel at a normal temperature of 300

37

Fig. 7 Variation of erosion rate with impact angle for both

carbon steel and 1.25 Cr-1 Mo-V steel at a normal temperature

(particle hardness with silica content 70%)

Fig. 8 Variation of erosion rate for Carbon and 1.25Cr-1Mo-V steel

as a function of impingement angle at normal temperature (30ºC) with

silica content 70 %

Fig. 9 Variation of erosion rate for 1.25Cr-1Mo-V and 12Cr- 1Mo-V

steel as a function of impingement angle at normal temperature

(30ºC) with silica content 70%

Fig. 10 Variation of erosion rate of 12 Cr-1Mo-v and Alloy 800steel

800 as a function of impingement angle at normal temperature (30ºC)

with silica content 70%

Fig. 11 Variation of erosion rate for Carbon and 1.25Cr-1Mo-V steelas a

function of impingement velocity at normal temperature (30ºC)

Fig. 12 Variation of erosion rate of Carbon Steel for two different

angularity

exponents ( K values) at normal temperature 30 C

Fig. 13 Variation of erosion rate for Carbon Steel and 12Cr-1Mo-V steel

as a

function of angularity exponent at normal temperature (30ºC)

38